We propose to explore a new position sensitive detector concept for positron emission tomography (PET). Although this new photon sensor concept could in principle be utilized in any high resolution PET cancer imaging system, we are focusing on breast cancer applications because there is particular potential for high impact. PET has shown promise for breast cancer imaging, but is not part of standard practice due to inadequate breast cancer specificity and sensitivity, and high cost. If successful, our developments will have impact on increasing the role of PET in breast cancer evaluation by addressing all of these issues. First, this project will impact the PET breast cancer sensitivity issue. As a long term goal of this program we will incorporate the new sensors into a camera dedicated to breast imaging of positron emitting tracers. This system will have < 1 mm spatial resolution and high signal to noise ratio in order to visualize tiny structures of high focal uptake as indication of early breast cancer. This camera will allow close-proximity breast imaging for optimal count sensitivity to help realize the desired high spatial resolution proposed. PET breast imaging poses particular challenges since typically the tracer may also be taken up significantly in the nearby heart, producing high background Compton scatter and random coincident photon rates and lower lesion to background contrast. The high energy and temporal resolutions and flexible orientation proposed will help to reduce photon scatter and random coincidence effects on image contrast. This system will also help breast cancer researchers to evaluate more specific breast cancer tracers and potential treatments by providing a dedicated breast imaging system that can rapidly generate high quality images. For this two-year project we will design, develop and evaluate a novel position sensitive detector array, and analyze potential designs for the full imaging system that incorporates these units. This array module pushes the limits of PET detector technology. This innovative device will have ultra-high (<1 mm) intrinsic coincidence spatial resolution, high 511 keV photon detection efficiency, and will maintain nearly perfect (>95%) scintillation light collection efficiency for optimal detector signals. The design uses scintillation crystals coupled in a novel manner to new, highly compact semiconductor photodetector arrays, and custom readout electronics.